Regarding a carbody of a railcar, conventionally known is a bodyshell using a predetermined-shape hollow section formed by extrusion using an aluminum alloy and the like as materials for the purpose of reducing the weight and the number of parts, improving the productivity, and the like. For example, in PTL 1, a closed cross section member is provided in a region of a cantrail between a roof block and a side block. The closed cross section member includes a hollow side joint member, a hollow shoulder member, and a hollow roof joint member, which are individually formed by extrusion, and the side joint member, the shoulder member, and the roof joint member are welded to one another. In the shoulder member, a bent point of a bent-shaped outside plate and a middle portion of a circular-arc inside plate are coupled to each other by a rib. However, each of the side joint member and the roof joint member is not provided with a rib for coupling.
For example, when a car enters or comes out of a tunnel, or when the car travels in the tunnel, a difference between internal pressure and external pressure of a carbody is generated by a fluctuation in air pressure outside the car, and therefore, external force called an airtight load acts on the bodyshell. Since railcars are increasing in speed in recent years, the strength of the bodyshell needs to be designed by adequately considering the airtight load. The bodyshell of PTL 1 is formed to have a cross section similar to a frame structure. Since each quadrangle of the cross section of the bodyshell may deform by the airtight load, bending resistance of the entire bodyshell resists against the deformation. Therefore, large bending stress acts on a corner portion of the bodyshell. Thus, the strength of the cantrail needs to be adequately increased.
PTL 2 discloses a double skin bodyshell including a cantrail formed to have a cross section similar to a truss structure by coupling a plurality of dividing wall portions to an outer side plate portion and an inner side plate portion such that a plurality of triangles are formed. Since the cantrail is formed such that a basic cross-sectional shape line obtained by virtually coupling apexes of the triangles of the cross section of the cantrail has a circular-arc shape, a largely bent portion is not formed, and therefore, local concentration of stress by a bending moment is prevented. In addition, since all of sections forming the triangles at the inner side plate portion and the outer side plate portion are made flat, a load transferred to the inner side plate portion and the outer side plate portion is received as in-plane stress, and therefore, out-of-plane deformation is prevented.